Controllable inductor apparatus



Nov. 13, 1962 w., D. GABOR CONTRQLLABLE INDUCroR APPARATUS Original Filed Jan. 19, 1955 )United States Patent Ofilice 3 064 219 CONTRGLLABLE INDUCTGR APEARATUS William D: Gabor, Norwalk, Conn., assigner to Trait Electronics Company, Inc., Wilton, Conn., a corpo- 2 Claims. (Cl. 336-155) This invention relates to variable frequency signal generators, and more particularly to the stabilization of variable-frequency generators capable of lbeing varied over wide ranges of frequency.

'It is possible, by the use of techniques and apparatus already known, to build highly stable oscillators whose frequency can be varied over a relatively limited range, for example 1.2 or 1.5 to one. The problem of stability, however, becomes increasingly troublesome as the operating range is increased in frequency.

One useful device for obtaining the wide variation in frequency includes as part of the tuning circuit inductances wound on ferrite core material. Such a tuning system is described Iby Dewitz in US. patent application Serial No. 213,548, filed March 2, 1951. However, this core material is sensitive to variations in tempera- `ture and exhibits substantial hysteresis effects. This latter characteristic causes the inductance of a winding wound on such a core to be a function of the past magnetic history.

The present invention is described as ern-bodied in apparatus in which the frequency of a wide-range oscillator is vcontrolled from a stable narrow-band oscillator. A discriminator is employed in the output circuit of the narrow-band oscillator, and a feed-back circuit is arranged so that the discriminator null point follows the output frequency of this oscillator. A saturable reactor type tuning system of the wide-band oscillator is controlled with the discriminator in such a way that changes in the discriminator null point control the output frequency of this latter oscillator. The feed-back control system eliminates instability introduced by the ferrite core of the saturable reactor.

vFIGURE l shows diagrammatically a variable-frequency signal generator embodying the invention;

FIGURE 2 illustrates the construction of a controllable inductor for use in the apparatus of FIGURE l; and

FIGURE 3 is an enlarged partial view of a modified form of controllable inductor suitable for use in the apparatus of FIGURE 1.

A narrow-band precision oscillator 2, which may be of any of `the well-known types such as a wienbridge oscillator, has a tuning knob 4 which may be manually operated, motor driven, or otherwise externally controlled to set the frequency of operation of the oscillator 2. The frequency range of this oscillator is selected for the particular application and may, for example, vbe variable from 100 to 130 kilocycles.

Two output terminals 6 and 8 of the highly-stable oscillator 2 are connected in series with windings 16 and 12 of two special transformers 14 and 16. The secondary windings 1S and 2G of these transformers are connected in parallel, respectively, with two capacitors 22 and 24 to form two resonant tank circuits of a stagger-tuned discriminator. The values of these elements are chosen so that the resonant frequency of the tank circuits are, respectively, slightly above and below the nominal operating frequency or discriminator null point.

One end of the secondary winding 18 of the transformer 14 is connected through a half-wave rectifier 26 and a lead 28 to an inpu-t terminal 30 of a direct current amplifier 32.

3,964,219 Patented Nov. 13, 1962 One end of the secondary winding 20 is connected, in a similar manner, through a half-wave rectifier 34 and a lead 36 to the other input terminal 38 of the amplifier 32.

The other ends of these windings 18 and 26 are connected together and to a lead 4f). In order to filter the pulsating direct current from the rectifier 26, a capacitor 42 and a resistor f4 are connected in parallel and between the leads 28 and 40. The current from the rectifier 34 is filtered in a similar manner by a parallel combination o-f a capacitor 46 and a resistor 43 connected between the leads 56 and 4f?.

The direct voltages developed across the load resistors i4 and 43 are in opposition and, because the values of the circuit elements in the two halves of the discriminator circuit are equal, no voltage is applied to the amplifier terminals 30 and 38 when the frequency o-f the signal from the oscillator 2 is at the center frequency of the discriminator.

The direct-current amplifier 32 may be any one of the well-known types, such as one using vacuum tubes, transistors, magnetic devices or the like. The output of this amplifier is connected by two leads 56 and 51 to a control winding 52 which is wound on an annular core 53 (see also FlGURE 2) of saturable ferrite maferial. Changes in current output of the amplifier 32, therefore control the magnetic permeability of the core The flux paths of transformers 14 and 16 are magnetically associated with and controlled by the core 53, as indicated by the broken line 54. One manner in which this relationship can be arranged is shown in FIGURE 2 where transformers 14 and 16 are shown as having generally U-shaped cores 53a and 53h, respectively, with their open ends located adajacent, and preferably in integral contact with, the core 53. Because the flux paths of the transformers 14 and 16 are cornpleted through the core 53, any changes in the permeability of the core 53 will also change the effective inductance of the windings 18 and 20.

Also magnetically coupled to core 53 is a signal winding 57, shown connected to a variable-frequency widerange oscillator 6ft. A capacitor 62 is connected in parallel with the signal winding 57 to form a frequencyselective circuit that controls the frequency of the signal produced by the oscillator 6l). The remainder of the circuitry of oscillator 6ft may be similar to any of the types well known in the art, and output terminals 63 and 64 are shown merely to indicate that a load can be connected to the output of thev oscillator.

In operation, the discriminator circuit is tuned to a null point at about the center of the frequency range of the narrow-range oscillator 2, for the condition when the iuput voltage to the amplifier 32 is zero. This condition results in a predetermined current flowing through the control winding 52, which in turn sets the inductance of the winding 57 at a particular value. Correspondingly, the output frequency of the oscillator 6) is set at a particular value, which will normally be midway in the range of output frequencies available from this oscillator.

If, due to temperature fluctuations or otherwise, there is a variation in the permeability of the core 53, there would normally be an accompanying shift in the output frequency of the oscillator 60. However, this same change in permeability will alter the inductances of the windings 1S and Zfl in such a way that the null point of the discriminator shifts from its original frequency. Since the output frequency of the oscillator 2 has not changed, there will be a resultant direct current potential at the output of the discriminator that is, between the leads 28 and 36. This potential is amplified by the amplifier 32, resulting in a variation in the current through the control winding 52. The direction of this current variation is such as to oppose the original fluctuation in permeability, and acts to restore the original magnetic condition of the core material. Therefore, the discriminator null point is reset to the frequency of the oscillator 2, and the output frequency of the oscillator 60 is stabilized by ythe feedback circuit which includes the discriminator and the amplifier. Y

If the output frequency of the oscillator 2 is varied, the discriminator circuit will again produce a direct current potential, which will be passed on by the amplifier 32 as a current variation in the control winding 52. This current variation will change the permeability of core 53, and hence change the inductance of the windings 18 and 20.

` This change will continue until the discriminator null point has been reset at the new output frequency of oscillator 2, and until the input voltage on the terminals 3f? and 3S of the amplifier 32 has been reduced nearly to zero. With this change in current through the winding 52, the inductance of the signal Winding 57 also changes, because of the saturation characteristics of the core 53. Therefore, the output frequency of the oscillator 60will vary in accordance with the inductance change. Because the inductance of such a saturable core device can be controllably varied over an extremely wide range, the output frequency of oscillator 6@ can be caused to cover a very broad band by varying the youtput frequency of oscillator 2 over a relatively narrow band. Y

The effectiveness of the stabilization system extends to any drift effects in the direct current amplifier 32. For example, if, because of changes in the emission of a vacuum tube in the amplifier :2,Y the output current through the winding 52 varies from its proper value,v

the immediaterr'esult will be a change iny the tuning of the discriminator null point. When this occurs, the discriminator will produce a direct current output voltage, since' the frequency of the oscillator 2 will no longer be on the null point. This output voltage will reset the output current passing through the winding 52 back towards its original rvalue and thereby stabilize the system.

should be formed of magnetically stable material so that the discriminator circuit is responsive only to changes in the control flux 70. These cores 53a and 53b may be formed of high Q powdered iron.

As described above, the DC. amplifier 32 is arranged to produce a predetermined current in the control winding 52 when the generator 2 is tuned to the null point Vof the discriminator so that the input at terminals 30 and 3S of the amplifier is zero. When the frequency of the generator 2 is above the null point, the amplifier 32 tends to increase the control current above this predetermined value, and when the frequency of the generator 2 is below the null point, the amplifier 32 tends to decrease the control current.

Alternately, where the amplifier 32 produces an output current which is zero where the discriminator output is. zero and reverses in response to a reversal of the discriminator output, then a source of bias magnetic flux may be used with the core 53. VThe bias source is shown hereV rIhe degree of stabilization is a function of feed-back Vgain around the loopincluding the discriminator, the amplifier 32, and the winding 52. This gain can be made 'sufficiently high to provide a significant improvement in the frequency stability of a wide range oscillator.

In circuits where the signal generator 2 is either manually or motor controlled, and the knobV 4 or similar indicator is used, an inner scale 66 may be associated with a pointerron the knob, with graduations indicating the frequency of the` generator 2, i.e., in the present case from 100 to 13() kilocycles. Around the scale 66 may be another' calibrated scale 68 which indicates theractual frequency of the oscillator 60 corresponding to each position of the knob 4. Y

As shown in FIGURE 2, the control winding 52 may be divided into two portions 52a and 5211 connected in series so that their magnetomotive forces are additive in .Y

the core 53, so'as to produce a control linxY flowing completely around the core 53 as indicated by the arrow 70. Alternatively, the control winding can be a single winding depending upon the configuration of the core 53.

The signal winding 57 preferably is divided into two portions 57a and S7b which are woundaround opposite edge portions of the core 53 and through a window or slotj72 formed inthe core53. The two halves 57a and 57h ofthe signal winding are connectedin series with their turns arranged so that the alternating signal flux produced bythe signals in the winding 57 travels around the slot and does not magnetically couple with the control winding 52. Thus, the inductance of the signal winding 57 quickly follows changes in the magnitude of thecontrol liux 76 due to changes in the control current in the control winding SZVbut otherwise the signal winding is isolated as a bias winding 74 connected through an adjustable resistor 76 to a bias current source 78. The bias flux is adjusted by resistor '76 to a value such as approximately to half saturate thefferrite material of the core 53. Then, the control flux 70 represents the net effect of the control and bias windings. Y Y

When the frequency of the generator 2 is above the null point, thecontrol current acts in the same direction as the bias current so as to increase the ux 70. When the frequency ofrthergenerator 2is belowV thenull point, the control current acts to oppose part of the bias linx,

reducing the control flux 7tlfrom its mean value. The

source of bias flux may be a permanent magnet associated 'withthe core S35;V Y y -In the controllable inductor shown in FIGURE 3, the

transformer 14 is arranged on an E-shaped core 53C of high Q powdered iron material. The winding 10y is Vsplit into two halves 10a and` 10b wound on the two outside legs of the core 53C andV connected in series.. The winding 18 is similarly divided into windings 18a and 18h. The transformer 16 has asimilar E-shapedrcore 53d and its windings 12 and20 are divided into windings 12a and 12b and 20a and Ztlb, respectively. Y

lAmong the advantagesof this arrangement is that the vtransformers 14 and 16 sample the permeability of the corev53- without any net voltage beingY induced in their windings due to changes in the signal flux 70, for any voltages induced in the halves of their windings cancel each other out. Conversely, there is no net resultant flux induced in the core 53 byV any currents flowingV through the'transformer windings.

As shown by the dotted arrows 80, the with the transformer Ywindings is confined tol the E-shaped cores 53cand 53d and to the portions of the ferrite core 53 immediately adjacent to them, thus not coupling with any other Awindings on the core 53.v

Embodiments of the invention othervthanv that shown in the drawings will be evidenttoY those skilledin theart.

For example, in some applicationsV it is possible to dispense with the direct current amplifier 32 by using an alternating current power amplifier immediately followingthe oscillator 2. Also for applications requiring high rates' of frequency variation, such as in sweep circuit, etc., a reactance tube or other device can be used withv the oscillator 2 to control the output frequency in accordance with input voltages; These and other variationsY are possible within the scope of the invention.

The present application is a division of application Serial No. 482,746, filedvJanuary 19, 1955, which issued as Patent No. 2,926,311, dated February 23, 196,0.

Further information4 in connection withthe construction of suitable types ofY saturable core devices can be obtained from the following copending applications, all in the name of Gerhard H. Dewitz:`

Serial No. 300,196, filed July 22, 1952, issued as Letters Patent No. l2,799,822, dated July 16, 1957;

flux associated Serial No. 300,746, filed July 24, 1952, issued as Letters Patent No. 2,802,185, dated August 6, 1957; and

Serial No. 310,341, filed September 18, 1952, issued as Letters Patent No. 2,886,789, dated May 12, 1959.

I claim:

l. A controllable inductor comprising a core portion of ferrite material, said ferrite core portion having an opening therein dividing said core portion into two parts, a signal winding on said core portion, said signal winding being divided into two halves, said halves being wound through said opening and around the respective parts of said ferrite core portion on opposite sides of said opening and connected in series aiding relationship with respect to magnetic ilux passing around said opening, at least one control ywinding associated with said core portion and arranged to vary its saturation to regulate the inductance of the signal winding, an E-shaped sensing core of highly permeable and magnetically stable material magnetically connected to three spaced regions of said core portion, said sensing core and the respective areas of said ferrite core portion between said three areas deining a pair of closed magnetic paths, and a sensing winding arranged on said E-shaped core and coupled to said pair of closed magnetic paths so that any current flowing in the sensing winding causes substantially equal amounts of flux in the two outside legs of said E-shaped core.

2. A controllable inductor comprising a core portion of magnetically saturable material dening a closed flux path, a signal winding on said core portion, a control winding for controlling the magnetic saturation of said core portion to regulate the inductance of the signal winding, an E-shaped sensing core of highly permeable and magnetically stable material connected to three spaced regions of said core portion, and a sensing winding arranged on said E-shaped core for providing equal amounts of ilux in the two outside legs of said E-shaped core.

References Cited in the file of this patent UNITED STATES PATENTS 2,519,426 Grant Aug. 22, 1950 2,708,219 Carver May 10, 1955 2,802,981 Hobart et al. Aug. 13, 1957 2,819,444 Walker Jan. 7, 1958 2,897,352 Smith-Vaniz July 28, 1959 FOREIGN PATENTS 467,911 Italy Dec. 28, 1951 

